Priority is claimed under 35 USC xc2xa7119(a) based on Japanese Patent Application Serial No. 206573 filed Jul. 6, 2001.
The present invention relates to a small engine for hand operated power tools wherein the air inlet to the carburetor is restricted to limit overrunning of the engine and further including a choke mechanism that enhances closure of the air inlet for starting and inhibits blow-back from the carburetor.
This invention relates to small engines such as used to power outdoor power tools including grass trimmers, hedge trimmers, brush cutters and the like. Such engines have carburetors that receive fuel and air and mix the fuel and air in a desired relationship which is thereafter conveyed to the combustion chamber of the engine in the form of vapor. The air is initially directed from an air cleaner to an air intake port of the carburetor. The passageway from the air cleaner to the air intake port in prior engines is at least the same size as the intake port of the carburetor.
A problem that is experienced during the use of such engines is the tendency by the operator to overrun or race the engine. Such overrunning is detrimental to the wear life of the engine and can also create safety concerns. Overrunning occurs when the throttle is held open in an unloaded condition. For example, in the use of any of the mentioned power tools, the tool may be used in a sweeping motion with a cutting or loaded condition occurring in one direction, and a non-cutting or non-loaded condition occurring in the other direction. A wide open throttle may be desired for the cutting or loaded condition and not in a non-cutting or non-loaded condition. In the loaded condition the speed is automatically slowed due to load resistance and a desired speed is maintained. The speed substantially increases when there is no load resistance being applied.
Whereas the operator has control over the throttle, it is common that at least part of the time the engine is racing faster than desirable or necessary.
One solution to this problem is the addition of an electrically controlling magneto-generator, which is the power supply for the engine""s spark plugs. The magneto-generator controls the ignition timing to establish a maximum rotation speed. Whereas this is a solution to the problem, it adds substantial costs to the engine.
A further problem that is addressed relates to the operability of the choke mechanism. The choke mechanism is used to selectively close the air inlet to the carburetor. Typically the choke mechanism comprises a plate that is slid (as by pivoting) into place over the air inlet. The result is a higher ratio of fuel to air mixture which aids in starting the engine. This choke mechanism can, over time, become loose, e.g., due to flexure, so that the desired closing of the air inlet is not achieved and starting is made more difficult. A still further problem is referred to as blowback. It can happen that in the process of pumping the fuel mixture from the carburetor to the crankcase chamber, a reverse pressure is created to blow the mixture back through the air cleaner and toward the operator. This is undesirable.
The three mentioned problems are resolved for the preferred embodiment at a location between the air cleaner and the air inlet to the carburetor. The mouth or opening that is the air inlet has a given dimension that becomes more restricted dimensionally as air travels through the carburetor. Such dimensional configuration is desirable if not necessary to achieve the required air fuel mix and vaporization. In prior engines, the large mouth or inlet opening permits a substantial volume of air to flow into and through the carburetor. As the engine runs faster, the piston correspondingly pumps a greater volume of fuel to the combustion chamber. The throttle needs to be at a given setting to produce a desired cutting action and does so without undue damage to the engine. That same setting when non-loaded will cause rapid or over running of the engine which is harmful to the engine and can create a safety concern.
The above problem is resolved by creating an airflow restriction at the mouth of the carburetor in the form of a plate that covers the mouth or inlet. An opening in the plate is sized to limit airflow into the carburetor. The suction or pumping that is created by the piston, at the point where it exceeds the limit imposed by the restriction, affects the balance of fuel to air mixture and such inhibits racing of the engine. The opening is sized to accommodate the anticipated work load for a particular engine. For example, it may be determined that a desirable cutting speed (under load) is X RPMs of the crankshaft and the opening is provided to accommodate piston reciprocation that produces that speed of the crankshaft. Above X RPMs the fuel to air mixture is increased, resulting in the slowing of the engine, i.e., the RPMs are substantially retained at that which produces the desirable cutting speed.
The choke plate flexure is alleviated by providing the choke plate, i.e., an end portion of the choke mechanism with an inset directed toward the carburetor and which functions as a stiffener. The inset is configured to fit, e.g., loosely, into the sized opening described above. As the end portion is pivoted and the inset portion is thereby aligned with the opening, the choke plate inset snaps into the opening and thus causing the plate to fully close off the opening. The choke plate inset has a small opening to allow a minimal air flow into the carburetor as desired for combustion of the fuel during the start-up mode. The snap-in action can be heard by the operator as a clicking sound to enable the operator to know that the choke plate is seated in the opening.
The blow-back of the fuel-air mixture is inhibited by a second arm or end portion (blow-back plate) that is provided on the choke mechanism. As the choke mechanism is pivoted to its non-choking position, the second end portion is aligned with but spaced from the opening. The air is thus allowed to flow around the plate and into the carburetor. However, in a blow-back situation, the direct path back through the carburetor and toward the air cleaner is impeded by the blow-back plate.